Parallel pass and filters having multiple negative feedback paths

ABSTRACT

An electrical frequency filter of the kind having several filter units covering adjacent passbands has negative feedback between adjacent units to tend to cancel unwanted frequencies.

Inventor Michael Hills Colchester, England Appl. No. 51,212

Filed June 30, 1970 Patented Oct. 5, 1971 Assignee National ResearchDevelopment London, England PARALLEL PASS AND FILTERS HAVING MULTIPLENEGATIVE FEEDBACK PATHS FieldofSearch 328/167, 165, 138, 307/229, 233,295; 329/142, 140; 330/21, 31, 107, 109; 324/77 B, 77 E; 333/70 R;

[56] References Cited UNITED STATES PATENTS 3,356,962 12/1967 Morgan330/109 X 3,411,093 11/1968 Gaylor 328/ X Primary ExaminerStanley T.Krawczewicz AllorneyCushman, Darby & Cushman 9 Claims, 5 Drawing Figs. T

US. Cl 328/167, ABSTRACT: An electrical frequency filter of the kindhaving 307/229, 324/77 E, 325/65, 325/458, 328/138, several filter unitscovering adjacent passbands has negative 330/109, 333/70 R feedbackbetween adjacent units to tend to cancel unwanted Int. Cl H03b l/04frequencies.

.2"/ 45 5 aurpz/rf x2 PARALLEL PASS AND FILTERS HAVING MULTIPLE NEGATIVEFEEDBACK PATHS BACKGROUND OF THE INVENTION In electrical frequencyfilters of the kind providing different outputs for the differentpassbands, it is difficult to provide filters which will block unwantedfrequencies without undulyattenuating the wanted frequencies.

The main object of the invention is to provide improved filter apparatusfor overcoming this difficulty.

SUMMARY OF THE INVENTION According to the present invention electricalfrequency filter apparatus comprises a plurality of band-pass filterunits covering different frequency bands and adapted to be fed from acommon input, each filter unit including a phase-inverting amplifier andconnections whereby at least some of the filter units are fed with thephase-inverted outputs from other filter units so as to effect at leasta partial cancellation of the other frequencies.

The invention is applicable, for example, to signal transmission systemssuch as speech frequency telegraph multiplex systems. It is alsoapplicable to doppler radar for separating the frequency bands of theindicating signals. In such a case there may be a filter for eachelement to be detected, the apparatus being so designed that theelements of the periodic table produce different frequency signals.

The invention is especially applicable to filter units for audio andsubaudio frequencies but is not limited to these frequencies and isapplicable also to higher frequencies. Preferably the input of eachfilter unit is fed with the outputs of all the other filter units.However, for some applications this may not justify the cost for thepurpose involved.

In other cases one or more unit filters on opposite sides of a passbandmay supply neutralizing signals.

BRIEF DESCRIPTION OF DRAWINGS In order that the invention may be moreclearly understood reference will now be made to the accompanyingdrawings, in which:

FIG. 1 shows in block form a three-filter unit arrangement,

FIG. 2 shows in greater detail an example of a single filter unit foruse in the arrangement of FIG. 1,

FIG. 3 shows an alternative single-filter unit, and

FIG. 4 shows how a bank of units may be arranged to provide alternativecharacteristics.

FIG. 5 shows an arrangement having a high frequency filter unit and alow frequency filter unit.

DESCRIPTION OF PREFERRED EMBODIMENTS In FIG. 1 the common electricalinput which extends over a wide frequency band is shown on the left;this is fed to three unity gain, phase-inverting amplifiers Al, A2 andA3, respectively, each of these amplifiers feeding an associated filterF1, F2 and F3, respectively. These filters cover adjacent frequencybands in the overall frequency band covered by the input signal.

As shown in the drawing each of the filters feeds a separate output;thus, filter Fl feeds output No. 1, filter F2 feeds output No. 2 andfilter F3 feeds the output No. 3. In addition, however, filter Flsupplies a separate output x1 which is fed as a compensating input tothe input terminals of the unity gain amplifiers A2 and A3 of the othertwo units. Similarly, the filter F2 supplies a compensating output A2 toamplifiers Al and A3 and filter F3 supplies a compensating output x3 toamplifiers A1 and A2.

Preferably each summing amplifier filter combination produces asubstantially complete phase reversal at the center frequency of thepassband concerned but there will not normally be complete phasereversal at, frequencies displaced from the center frequency of thepassband. It follows, therefore, that if the input signal contains afrequency at the center frequency of the filter F2, then thecompensating output x2 will be of equal amplitude and opposite phase tothe input. As the output x2 is supplied to the inputs of the amplifiersA1 and A3, there will be zero transmission of this frequency through thefirst and third filter units.

A similar condition arises for signals at the center frequencies of thefilters F1 and F3.

For frequencies near to but not quite at the center frequencies of theadjacentbands, the output from the appropriate filter will not cancelexactly with the signal frequency; however, there will be some reductionof the signal passing through the other amplifiers to which thisfrequency is fed. It follows, therefore, that the effect of theinterconnection is to introduce zeros of transmission of the frequencycharacteristic of an individual filter at all center frequencies of theother filters and a general reduction of frequencies outside the passband concerned. FIG. 2 shows a single-filter unit, assumed to be unit I,forming a summing and inverting circuit in which the main input and thecompensating inputs x1 and x2 from the other units are fed in throughresistors R1 to a high gain inverting amplifier IA (having a gain ofinfinity or approaching infinity). A resistor R2 is connected across theamplifier IA, thus giving a summing and inverting amplifier. The outputfrom IA is fed through a filter circuit fonned by L, C and R3 to theoutput terminal.

FIG. 3 shows an alternative arrangement in which the filter circuit ofFIG. 2 is replaced by an amplifier AMP having a fixed positive gain Kand a network formed by C3, C4 and R4.

It will be appreciated that with such arrangements the filters may bemade of a relatively simple construction and furthermore it is possibleto predict their characteristics to a high degree of accuracy.

As a guide to the design of the filters, if the voltage transferfunction of the nth unit filter is A then in a fully interconnectedsystem, the modified transfer function f, may be shown to be where N isthe number of unit filters.

A suitable design procedure is to find a selection of transfer functionsA,, such that f, has a (or approximates to) some desired characteristicwithin the effective passband of the unit. The resulting stopbandbehavior may then be computed and in a wide range of practicalapplications this is a useful improvement over what a single unit ofsimilar complexity could achieve.

By using computerized design methods it is possible to ob tain transferfunctions of the unit filters such that they have a specified passbandamplitude characteristic when they are interconnected.

By tabulating the resulting stopbands, the improvement over a comparabledesign of a filter with the same complexity may be seen.

In some arrangements additional units may be added at each end of thefrequency band in order to improve the performance of the end units. Theamplifiers may be transistor amplifiers.

In the arrangement above described there is an upper limit to the ratioof bandwidth to the spacing between the center frequencies of individualfilters. This ratio limit may be in- A particular application of theinvention is to a system which includes a low-pass and high pass filtercombination, as shown in FIG. 5. The output of the low-pass filterL.P.F. is fed through a mixer M2 to the input of the high pass filterH.P.F. is fed through a mixer M1 to the input of the low-pass filterL.P.F.

I claim:

Electrical frequency filter apparatus comprising:

a plurality of band-pass frequency filter units covering differentfrequency bands and adapted to be fed from a common input,

each filter unit including a phase-inverting amplifier and connectionswhereby at least some of the filter units are fed with thephase-inverted outputs from other filter units so as to effect at leasta partial cancellation of the other frequencies.

2. Electrical frequency filter apparatus comprising:

a plurality of band-pass frequency filter units covering adjacentfrequency bands in an overall frequency range and adapted to be fed froma common input but having separate outputs,

each filter unit including a substantially unity gain phase-invertingamplifier and means whereby each filter unit is also fed with thephase-inverted outputs from at least one frequency filter unit coveringan adjacent frequency band.

3. Electrical frequency filter apparatus comprising:

a plurality of band-pass frequency filter units covering adjacentfrequency bands in an overall range and adapted to be fed from a commoninput but having separate outputs,

each filter unit including a substantially unity gain phase-invertingamplifier,

means whereby each filter unit is also fed with the phase-invertedoutputs from each of the other unit filters to tend to cancel the otherfrequencies in the unit concerned.

4. Electrical frequency filter apparatus, as claimed in claim 3,comprising means for connecting the outputs of the unit filters in pairsto obtain required characteristics.

5. Electrical frequency filter apparatus, as claimed in claim 3,comprising additional frequency filter units covering frequencypassbands outside each end of the overall frequency range, each unitincluding a unity gain phase-reversing amplifier and means connectingthe output of each of said units to an input of the unit filter at thecorresponding end of the overall frequency range.

6. Electrical frequency filter apparatus comprising:

a high-frequency band-pass filter unit including a substantially unitygain phase-inverting amplifier,

a low-frequency filter band-pass filter unit including a substantiallyunity gain phase-inverting amplifier,

said filter units having a common input but separate outputs,

means where the output from the high-frequency unit is also fed to aninput of the low-frequency filter to tend to cancel high frequencies inthe low-frequency unit output and means whereby the output of thelow-frequency unit is also fed to an input of the high-frequency unit totend to cancel low frequencies in the high-frequency unit output.

7. In a signal transmission system electrical frequency filter apparatuscomprising:

a plurality of band-pass frequency filter units covering differentpassbands in the frequency range of the signals said filter units beingfed from a common input but having separate outputs,

each filter unit including a substantially unity gain phase-invertingamplifier,

means whereby the output of each filter unit is also fed in phaseopposition to an input of at least an adjacent passband unit to tend tocancel unwanted frequencies.

8. In a doppler radar system electrical frequency filter apparatuscomprising:

a plurality of band-pass frequency filter units covering differentpassbands in the frequency range of the received fre uency signals said1 ter units being fed from a common input but having different outputscorresponding to difl'erent ranges,

said filter units each including a substantially unity gainphase-inverting amplifier,

means whereby the output of each filter unit is also fed in phaseopposition to an input of at least an adjacent passband filter unit totend to cancel unwanted frequencies in the output of the filter unit.

9. An analyzing apparatus in which the elements detected aredistinguished by signals of different frequencies, frequency-analyzingapparatus comprising:

a plurality of band-pass frequency filter units covering differentpassbands in the overall frequency range of the signals,

said filter units being fed from a common input but having differentoutputs,

said filter units each including a substantially unity gainphase-inverting amplifier, and

means whereby the phase-inverted output of each filter unit is also fedto an output of at least an adjacent passband filter unit to tend tocancel unwanted frequencies in the output of the filter unit.

2. Electrical frequency filter apparatus comprising: a plurality ofband-pass frequency filter units covering adjacent frequency bands in anoverall frequency range and adapted to be fed from a common input buthaving separate outputs, each filter unit including a substantiallyunity gain phase-inverting amplifier and means whereby each filter unitis also fed with the phase-inverted outputs from at least one frequencyfilter unit covering an adjacent frequency band.
 3. Electrical frequencyfilter apparatus comprising: a plurality of band-pass frequency filterunits covering adjacent frequency bands in an overall range and adaptedto be fed from a common input but having separate outputs, each filterunit including a substantially unity gain phase-inverting amplifier,means whereby each filter unit is also fed with the phase-invertedoutputs from each of the other unit filters to tend to cancel the otherfrequencies in the unit concerned.
 4. Electrical frequency filterapparatus, as claimed in claim 3, comprising means for connecting theoutputs of the unit filters in pairs to obtain required characteristics.5. Electrical frequency filter apparatus, as claimed in claim 3,comprising additional frequency filter units covering frequencypassbands outside each end of the overall frequency range, each unitincluding a unity gain phase-reversing amplifier and means connectingthe output of each of said units to an input of the unit filter at thecorresponding end of the overall frequency range.
 6. Electricalfrequency filter apparatus comprising: a high-frequency band-pass filterunit including a substantially unity gain phase-inverting amplifier, alow-frequency filter band-pass filter unit including a substantiallyunity gain phase-inverting amplifier, said filter units having a commoninput but separate outputs, means where the output from thehigh-frequency unit is also fed to an input of the low-frequency filterto tend to cancel high frequencies in the low-frequency unit output andmeans whereby the output of the low-frequency unit is also fed to aninput of the high-frequency unit to tend to cancel low frequencies inthe high-frequency unit output.
 7. In a signal transmission systemelectrical frequency filter apparatus comprising: a plurality ofband-pass frequency filter units covering different passbands in thefrequency range of the signals said filter units being fed from a commoninput but having separate outputs, each filter unit including asubstantially unity gain phase-inverting amplifier, means whereby theoutput of each filter unit is also fed in phase opposition to an inputof at least an adjacent passband unit to tend to cancel unwantedfrequencies.
 8. In a doppler radar system electrical frequency filterapparatus comprising: a plurality of band-pass frequency filter unitscovering different passbands in the frequency range of the receivedfrequency signals said filter units being fed from a common input buthaving different outputs corresponding to different ranges, said filterunits each including a substantially unity gain phase-invertingamplifier, means whereby the output of each filter unit is also fed inphase oppoSition to an input of at least an adjacent passband filterunit to tend to cancel unwanted frequencies in the output of the filterunit.
 9. An analyzing apparatus in which the elements detected aredistinguished by signals of different frequencies, frequency-analyzingapparatus comprising: a plurality of band-pass frequency filter unitscovering different passbands in the overall frequency range of thesignals, said filter units being fed from a common input but havingdifferent outputs, said filter units each including a substantiallyunity gain phase-inverting amplifier, and means whereby thephase-inverted output of each filter unit is also fed to an output of atleast an adjacent passband filter unit to tend to cancel unwantedfrequencies in the output of the filter unit.